Information communication equipment, such as, in particular, personal compact information equipment typified by mobile communication terminal devices, such as portable terminals, is becoming increasingly common, and in such circumstances, further higher performance, such as higher integration, higher speed, and lower power consumption, is required for memory elements, logic carriers, and the like constituting the information communication equipment. For example, increases in density and capacity of nonvolatile memories are important as complementary technologies for magnetic recording devices including movable parts, such as magnetic hard disc drives. In other words, since it is difficult to reduce the size, increase the speed, and reduce the power consumption in devices including movable parts, the role of nonvolatile memories is becoming increasingly important in fields where carryability and portability are regarded as being important.
Semiconductor flash memories, ferroelectric nonvolatile memories (FeRAMs), and the like are commercially used as nonvolatile memories, and research and development is being carried out in order to improve the performance even more. Recently, MRAMs (magnetic random access memories) utilizing the tunnel magnetoresistance (TMR) effect have been the focus of attention as nonvolatile memories using a magnetic material (see, for example, “Naji et al. ISSCC2001”).
The principle of operation of an MRAM will now be briefly explained. This memory device includes many minute storage carriers (magnetic storage carriers) made of magnetic materials. The MRAM has a structure in which wiring for regularly arranging such storage carriers and for allowing access to the respective storage carriers is provided, so that magnetic information recording can be performed.
For example, in an arrangement in which two pairs of parallel conducting wires cross each other to form a lattice structure and in which magnetic storage carriers (memory cells) are disposed in positions corresponding to respective lattice points of the parallel conducting wires, information writing is performed by controlling the magnetization of the magnetic materials by using a magnetic field (combined current magnetic field) generated by allowing predetermined current to flow in a conducting wire (word line) arranged at one end of each of the magnetic storage carriers and in a conducting wire (bit line) for reading arranged at the other end of each of the magnetic storage carriers. Generally, depending on the direction of magnetization of the magnetic materials, information of a logical value of “0” and information of a logical value of “1” are differentiated from each other and stored. Also, for information reading, a cell is selected by using an element such as a transistor, and the direction of magnetization of a magnetic material constituting the cell can be extracted as a voltage signal in accordance with the galvanomagnetic effect. Each of the cells has a basic film structure, for example, such as a three-layer structure (a ferromagnetic tunnel junction, that is, a magnetic tunnel junction=“MTJ”) including a ferromagnetic material, an insulating material, and a ferromagnetic material. Using one ferromagnetic layer as a fixed reference layer and using the other ferromagnetic layer as a storage layer causes the direction of magnetization of the storage layer to correspond to a voltage signal in accordance with the TMR effect.
It is obvious that write operation also needs selection of a desired cell to store information. A method for selecting a cell will now be described.
In general, it is known that, when a magnetic field in the direction opposite to magnetization is applied in the direction of the easy magnetization axis of a ferromagnetic material, the magnetization is reversed to the direction of the applied magnetic field at a critical value±Hsw (this is called a “reversal magnetic field”). The value of the reversal magnetic field can be theoretically calculated from the minimum energy condition. It is also known that, in a case where the magnetic field is applied not only in the direction of the easy magnetization axis but also in the direction of the hard magnetization axis, the absolute value of the reversal magnetic field is reduced. The value of the reversal magnetic field in this case can also be calculated from the minimum energy condition. When the magnetic field applied in the direction of the hard magnetization axis is represented by “Hx” and the reversal magnetic field at that time is represented by “Hy”, the relationship between the applied magnetic field and the reversal magnetic field is represented as follows:Hx2/3+Hy2/3=Hc2/3.
A curved line (for example, see FIGS. 4 and 7) represented by the above mathematical expression on the Hx-Hy plane having two orthogonal axes, Hx and Hy, is called an asteroid curve. Using the asteroid curve is convenient for explaining the method for selecting a cell. Here, “Hc” represents coercive force.
In MRAMs having a structure in which a magnetic field generated by a recording word line is approximately equal to the direction of the easy magnetization axis, the magnetization is reversed by the magnetic field generated by the word line to perform recording. Since a plurality of cells are regularly spaced from the recording word line, if current that generates a field larger than the reversal magnetic field flows in the recording word line, recording is also unintentionally performed in other cells (positionally equivalent cells). Thus, when current flows in a bit line corresponding to a cell desired to be selected (selected cell) to generate a magnetic field in the hard magnetization field, recording can be performed without affecting the other equivalent cells by utilizing a reduction in the reversal magnetic field of the selected cell. In other words, when the reversal magnetic field of the selected cell in a case where current flows in the bit line is represented by “Hc(h)” and the reversal magnetic field in a case where the magnetic field generated by the bit line is 0 (in other words, current does not flow) is represented by “Hc(0)”, if the magnetic field “H” generated by the current of the recording word line is set so as to satisfy the relationship of “Hc(h)<H<Hc(0)”, only a desired cell is selected to perform recording. (In other words, this is because that, although magnetization reversal occurs in the selected cell since the magnetic field H of the selected cell is larger than Hc(h), magnetization reversal does not occur in the equivalent cells since the magnetic field H of the equivalent cells is smaller than Hc(0).)
As described above, MRAMs arranged using a plurality of recording carriers including magnetic materials have advantages, for example, as described below. Thus, such MRAMs are regarded as being promising as future nonvolatile memories.
1) To be nonvolatile and be capable of nondestructive reading and random access;
2) To be rewritable a large number “N” of times (N>1015);
3) To operate at high speed (processing speed<5 nanoseconds);
4) To be free from soft errors; and
5) To have excellent process uniformity because they are formed only by wiring processing after production of MOS elements.
In particular, MRAMs have better performance than flash memories in points 1 to 3 mentioned above. Also, MRAMs are superior to the above-mentioned FeRAMs in point 5. MRAMs are expected to be used as memories capable of satisfying both a higher level of integration, similar to the DRAM level, and a speed similar to the SRAM level. Such MRAMs have the potential to replace all system LSI consolidated memories.
Incidentally, the use of current for performing information writing may be an obstacle to an increase in the performance of MRAMs.
In general, wiring has an upper limit of an allowable current density, and a current exceeding the upper limit is likely to cause deterioration and breaking due to electromigration. In particular, in accordance with an increase in the level of integration and a reduction in the width of wiring, a critical current value that is allowable to flow in the writing line is reduced; thus reducing the size of a magnetic field generated by the current. Thus, although the coercive force (Hc) in a storage area of a storage carrier must be reduced, a technological difficulty occurs because, in general, the coercive force of a storage area tends to be increased in accordance with a reduction in the size of a cell. Reconsideration and investigation of fundamental items, for example, such as structural design of elements and selection of materials, are unavoidable.
Accordingly, an object of the present invention is to raise the upper limit of the current density of wiring, without significantly changing the material, structure, and the like, in order to deal with higher integration of storage elements constituting a magnetic memory device and miniaturization of wiring.